The invention relates to pulp production processes which are based on organic chemicals and where herbaceous plants, particularly grain straw, are used as raw material.
Using existing processes for producing chemical cellulose, cellulose pulp can be produced in an economical and environmental friendly manner when wooden raw materials are employed. However, one has not, either technically or economically, succeeded in producing cellulose that would satisfy the environmental requirements from herbaceous plants, i.e. non-wood materials.
To eliminate the environmental effects of chemical pulp production, ‘organosolv’ processes based on organic cooking solvents have been devised. The organosolv processes are based on the utilization of organic solvents, typically organic acids, in delignification. Several of these processes are based on sulphur-free and chlorine-free chemicals, which can be regarded as the requirement for the industrial implementation of closed circulation of chemicals. Such processes are described e.g. in WO 96/26403 (Chempolis Oy) and WO 00/60160 (Chempolis Oy).
Non-wood material is most preferably delignified in acid conditions. In that case the silicon included in the non-wood material is in a very poorly soluble form and does not thus cause problems in the recovery of chemicals. In alkaline cooking processes, instead, silicon dissolves into the cooking liquor and causes problems. For this reason, the most environmental friendly and economical way of implementing delignification is to carry it out by organic solvents in acid conditions. It is particularly preferable to utilize a process where chemicals to be used in the cooking are formed in the process itself.
Several organosolv processes have been reported to produce furfural during the production of cellulose. In general, it has been argued that production of furfural in acid conditions impedes delignification because furfural may react with lignin and other compounds of the plant material, forming caramel-like polymers. This was noticed e.g. in an organosolv process based on acetic acid (Zil'bergleit, M. A. & Glushko, T. V., Products from the Polymerization of Furfural and Hydroxymethylfurfural in Acetic Acid, Khimia Drevesiny (Riga), 1991, no 1, 66–68) and in acid preliminary hydrolysis of the M. A., New Process of Preliminary Hydrolysis, Bumaznaja promyslennost, 1982, no 9, 12–13). It has been found that furfural also reacts in the sulphite cooking, causing pulp blackening (Oblak-Ramer, M., Budin, D. & Lipic, B., Concerning the basics of condensation of lignins in magnesium bisulphite digestion. Part 1. Influence of processing time, temperature and thiosulphate, Zellstoff und Papier 40 (1991), no. 1, 10–13). Utilization of furfural in the actual delignification process has not been described in the literature related to the art; instead, furfural has usually been separated from the process. For example, it has been suggested that the furfural separated from the Alcell process (WO 93/15261, Lora et al.) and the Formacell process (Lehnen, R., Saake, B. & Nimz, H. H., Furfural and Hydroxymethylfurfural as Byproducts of FORMACELL Pulping, Holzforschung 55 (2001), no. 2, 199–204) be utilized as a commercial product.
It has been found earlier that high-quality cellulose pulp can be produced advantageously using a mixture of formic acid, acetic acid and water in delignification (WO 99/57364, Chempolis Oy). Surprisingly, it has now been found that pulp can be produced from a delignification mixture based on organic acids using also furfural.